Heat oil in a pan and add mustard seeds. Once they start to pop, add the remaining tadka ingredients and sauté for a few seconds.
Add potatoes, if using, and fry for a couple minutes. Then add the peas (if using) and methi leaves and fry for a minute, then cover and cook until done.
Now stir in the bhath powder, tamarind paste and salt to taste. Cover and cook for a couple more minutes.
Stir in the coconut and switch off stove.
Add the cooled rice to this and mix well until the rice is completely coated with the methi masala.
Taste to adjust seasonings and finish with lemon juice.